Ceramic overvoltage protection device having low capacitance and improved durability

ABSTRACT

Provided is an improved overvoltage protection element. The overvoltage protection devices comprises at least one ESD protection couple comprising discharge electrodes in a plane, a gap insulator between the discharge electrodes, an overvoltage protection element parallel to the planar discharge electrodes wherein the overvoltage protection element comprises a conductor and an secondary material. The overvoltage protection element also comprises a primary insulator layer between the discharge electrodes and overvoltage protection element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of pending U.S. patentapplication Ser. No. 16/195,159 filed Nov. 19, 2018 which isincorporated herein by reference in its entirety.

BACKGROUND

The present invention is related to a ceramic overvoltage protectiondevice which provides improved control over the trigger voltage, hasreduced leakage current and which does not break down with repeatedovervoltage pulses.

There is an ongoing need in modern electronics for increased protectionof semiconductors, integrated circuits and other components fromElectro-Static Discharge (ESD) and fast electronic transients. ESD canreach over 30 kV which is well beyond that which a core processor iscapable of withstanding. ESD is a primary cause for the failure ofintegrated circuits and this is a particular problem with respect tominiaturization of electronics wherein suitable protection has provendifficult to provide reliably.

The necessity for ESD protection has increased that portion of theavailable space in a device devoted to ESD protection. This utilizationof space, for other than device functionality, is contrary to theconstant desire for miniaturization of components and devices. Inaddition to the space consumption of on-chip ESD protection devices theyalso inhibit the speed and quantity of data that can be processed.Therefore, there is a desire for off-chip protection and, particularly,discrete components which can provide ESD protection, without loss ofprocessing speed, and which do not consume valuable space within thecircuit design or in the device.

Multilayer Ceramic Capacitor (MLCC) devices have an established role inESD protection. The higher capacitance levels of an MLCC absorb chargefrom an ESD or transient event. Unfortunately, high capacitancecompromises signal integrity in high speed data applications. If thecapacitance of the MLCC is reduced its ability to absorb the ESD ortransient event is lowered because the charge absorbed, Q is dependenton the capacitance C, and voltage V; Q=CV. It can therefore be seen thatif capacitance is lowered by 50% the voltage that the capacitor has towithstand has to increase by 50% to maintain the same capability toabsorb the coulombic charge, Q. In practice it is not possible toincrease the voltage withstanding capability of low capacitance MLCC tothis extent so their capability is reduced. Furthermore, when protectivedevices such as varistors are employed the leakage current to ground,while the device is inactive, can be a major drain on the standby lifeof battery powered devices. Any parasitic drain on a battery isundesirable and therefore protection devices with high leakage and highcapacitance are less desirable.

To meet the demands of modern circuitry an ideal ESD protection devicewould preferably have a low capacitance with negligible leakage currentto ground in its inactive state. When exposed to elevated voltage orcurrent, due to an ESD event, the ESD protection device should respondrapidly, such as <1 ns, to divert potentially harmful transient energyto ground via a low resistance path. After the ESD event has subsidedthe ESD protection device should revert to its previous inactive state.Furthermore, the ESD protection device should be able to withstandmultiple ESD or transient events while returning to near the pre-eventcapacitance and near pre-event leakage current characteristics of itsinitial inactive mode. Meeting all of these criteria, particularly withhigh voltage applications, has proven to be difficult as evidenced bythe lack of a suitable ESD protection devices available to designers ofmodern circuitry.

Provided herein is an improved ESD protection device which meets theadvanced demands of modern circuitry.

SUMMARY OF THE INVENTION

The invention is related to an improved overvoltage protection deviceand a method for forming the improved overvoltage protection device.

It is an object of the invention to provide an overvoltage protectiondevice with low capacitance, low leakage current and the ability towithstand many ESD pulses.

A particular feature of the invention is the ability to manufacture theovervoltage protection device using standard manufacturing techniquesused for the manufacture of MLCC capacitors.

Another feature of this invention is to allow high ESD protection to becombined with a low capacitance multilayer ceramic capacitor such thatthe combined device can protect from high transient voltages whilstretaining capacitance for signal filtering while occupying only aminimum amount of available space.

These and other embodiments, as will be realized, are provided in anovervoltage protection device. The overvoltage protection devicescomprises at least one ESD protection couple comprising dischargeelectrodes in a plane, a gap insulator between the discharge electrodes,an overvoltage protection element parallel to the planar dischargeelectrodes wherein the overvoltage protection element comprises aconductor and an secondary material. The overvoltage protection elementalso comprises a primary insulator layer between the dischargeelectrodes and overvoltage protection element.

Yet another embodiment is provided in a method for forming anovervoltage protection device. The method comprises the steps:

-   forming at least one first layer comprising a secondary insulator    precursor and an overvoltage protection element precursor on a    primary insulator precursor;-   forming at least one second layer on an internal insulator precursor    comprising a pair of discharge electrodes and a gap insulator    precursor between the discharge electrodes;-   forming a stack comprising at least one first layer on at least one    second layer in registration with the over overvoltage protection    element precursor overlaying the gap insulator precursor; and-   heating the stack to form a laminated stack comprising:-   discharge electrodes in a plane;-   a gap insulator between the discharge electrodes in the plane;-   an overvoltage protection element parallel to the discharge    electrodes; and-   a primary insulator layer between the discharge electrodes and    overvoltage protection element.

Yet another embodiment is provided in a dual-function overvoltageprotection device. The overvoltage protection device comprises at leastone ESD protection couple comprising discharge electrodes in a plane, agap insulator between discharge electrodes, an overvoltage protectionelement parallel to the discharge electrodes and a primary insulatorlayer between the discharge electrodes and overvoltage protectionelement. The dual-function overvoltage protection device furthercomprises a capacitive couple.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a cross-sectional schematic view of an embodiment of theinvention.

FIG. 2 is a cross-sectional schematic view of an embodiment of theinvention.

FIG. 3 is a cross-sectional schematic expanded view of an embodiment ofthe invention.

FIG. 4 is a cross-sectional schematic expanded view of an embodiment ofthe invention.

FIG. 5 is a top schematic view of an embodiment of the invention.

FIG. 6 is a flow-chart representation of an embodiment of the invention.

FIGS. 7 and 8 are graphical representations illustrating the advantagesof the invention.

DESCRIPTION

The instant invention is related to an improved ESD protection devicewhich has a low capacitance and low leakage current in the inactivestate, has a rapid response time in the presence of ESD events, andwhich can withstand many ESD events without compromise of capacitance,leakage current or functionality. More specifically, the presentinvention is related to a layered structure comprising a discreteinsulator layer as a laminate between discharge electrodes and anovervoltage protection element. The structure provides for optimizationof the trigger voltage which is the threshold voltage at which transientenergy is diverted to electrical ground. The instant invention is alsorelated to an ESD protection device which also comprises a capacitivecouple suitable for signal filtering below the threshold voltage whereinabove the threshold voltage the transient energy is diverted toelectrical ground.

During normal operation the ESD protection couple of the ESD protectiondevice is passive, or does not provide any functionality to the circuit,so by design the ESD protection couple is expected to exhibit lowcapacitance so that high speed signal distortion can be minimized.However, during an ESD event the ESD protection couple effectively actsas a switch allowing excess voltage to be dispersed to electricalground.

The invention will be described with reference to the figures forming anintegral, non-limiting, component of the disclosure. Throughout thedescription similar elements will be numbered accordingly.

An embodiment of the invention will be described with reference to FIG.1 wherein a ESD protection couple, 2, is illustrated in cross-sectionalschematic view. In FIG. 1, discharge electrodes, 12, which arepreferably coplanar, are separated in the plane by a gap insulator, 14.A primary insulator layer, 18, extends in a planar manner over thedischarge electrodes forming a laminate structure. An overvoltageprotection element, 16, is separated from each discharge electrode bythe primary insulator layer, 18. A secondary insulator, 20, preferablysurrounds the overvoltage protection element within the layer comprisingthe overvoltage protection element. In a preferred, but non-limitingembodiment, the gap insulator, primary insulator layer and secondaryinsulator are the same material for manufacturing convenience. The ESDprotection couple functions by conducting the pulse energy between thedischarge electrodes, via the overvoltage protection element, during thetransient event wherein the pulse energy passes through the primaryinsulator layer at each transit between the overvoltage protectionelement and discharge electrode. In a particularly preferred embodimentthe width of the overvoltage protection element, 16, is no more than thewidth of the gap insulator, 14, thereby minimizing the overlap of theovervoltage protection element and the discharge electrodes, 12.

An embodiment of the invention is illustrated in cross-sectionalschematic view in FIG. 2. In FIG. 2, the ESD protection device, 10,comprises multiple ESD protection couples, 2, in a layered or laminatearrangement. An optional but preferred internal insulator, 24, may bebetween adjacent ESD protection couples. An external insulator, 22, isexternal to all ESD protection couples. The external insulator andinternal insulator are preferably the same for manufacturingconveniences without limit thereto. External terminations, 26, inelectrical contact with the discharge electrodes, 12, allow the ESDprotection device to be electrically connected to a circuit as would berealized by those of skill in the art.

In a further embodiment of this invention the aforementioned ESDprotection device may be combined with a capacitive couple to form adual-function ESD protection device. The capacitive couple may consistof at least 2 overlapping electrodes of opposite polarity or thecapacitive couple may employ a floating electrode. When a capacitivecouple is incorporated it is desirable to minimize the overlap areabetween the discharge electrode and overvoltage protection element toreduce the stray capacitance thereby maintaining a low capacitance forthe combined component. For convenience of manufacture the primaryinsulator and dielectric of the capacitor element is the same. In thiscase it is essential that the thickness of the dielectric separating theelectrodes of the capacitor exceeds the primary insulator thickness.

An embodiment of the invention is illustrated in cross-sectionalexpanded view in FIG. 3. In FIG. 3, a dual-function ESD device, 11, isillustrated. The dual-function ESD device comprises up to n ESDprotection couples, 2, wherein n is the number of ESD protection couplesin the device. At least m capacitive couples, 4, are illustrated whereinm is the number of capacitive couples in the dual-function ESD device.The capacitive couples comprise parallel internal electrodes, 25, ofalternating polarity wherein adjacent internal electrodes terminate atdifferent external terminations, 26, and adjacent internal electrodesare separated by a capacitor ceramic, 23. The internal insulator, 24,may be between adjacent ESD protection couples, 2, which is preferred.It is preferable that capacitor ceramic, 23, is in alternating layersbetween adjacent internal electrodes, 25, such that each set of adjacentinternal electrodes forms a capacitive couple and therefore the numberof capacitive couples functions as a single capacitor. The capacitorceramic, 23, external insulator, 22, internal insulator, 24, gapinsulator, 14, and primary insulator layer, 18, are preferable the samematerial for manufacturing convenience. When the primary insulator layerand capacitor ceramic are the same composition it is preferable that theseparation of the overvoltage protection element, 16, from the dischargeelectrodes, 12, represented as T_(OVP), is less than the separationdistance between adjacent internal electrodes, 25, represented as T_(C).A smaller separation distance in the ESD protection couples, 2, than inthe capacitive couples, 4, prevents permanent electrical breakdown ofthe capacitative couple upon exposure to an ESD event. More preferably,T_(C) is over two times larger that T_(OVP).

An embodiment of the invention is illustrated in cross-sectionalexpanded view in FIG. 4. In FIG. 4, a dual-function ESD device, 11, isillustrated which is similar to the device illustrated in, and describedrelative to, FIG. 3. In FIG. 4 the dual-function ESD device comprises mfloating electrode capacitive couples, 41. The floating electrodecapacitive couple comprises coplanar internal electrodes, 25, ofopposite polarity with each terminating to an opposite externaltermination, 26. A floating electrode, 31, is parallel to the coplanarinternal electrodes and separated from the plane of the coplanarinternal electrodes by capacitor ceramic, 23. When the primary insulatorlayer and capacitor ceramic are the same composition it is preferablethat the separation of the overvoltage protection element, 16, from thedischarge electrodes, 12, represented as T_(OVP), is less than theseparation distance between the plane of the coplanar internalelectrodes, 25, and the floating electrode represented as T_(FLO). Asmaller separation distance in the ESD couples, 2, than in thecapacitive couples, 41, promotes temporary electrical breakdown of theESD couples in preference to permanent electrical breakdown of thecapacitative couples.

It is preferred that the capacitance of the ESD protection device is atleast 0.1 to no more than 23,000 pF. When the ESD protection devicesfurther comprises at least one capacitive couple it is preferable thatthe ESD protection device have a capacitance of at least 100 pF and morepreferably at least 1000 pF. When the ESD protection device does notcomprise a separate capacitive couple it is preferable that thecapacitance of the ESD protection device be no more than 100 pF,preferably no more than 10 pF and even more preferably no more than 2pF.

The ESD protection device may be a 2-terminal device wherein a separateexternal terminal is in electrical contact with adjacent co-planerdischarge electrodes, as illustrated in FIG. 2. Alternatively, theovervoltage protection elements may terminate and be in electricalcontact with secondary external terminations collectively, orindependently, to form multi-terminal devices such as a 4-terminaldevice, as illustrated, without limit thereto, in FIG. 5. In FIG. 5,secondary external terminations, 28, may be in electrical contact withthe same ESD protection couple, separate ESD protection couples, or acapacitive couple as described relative to FIGS. 3 and 4.

The number of ESD protection couples in an ESD protection device is notparticularly limited. At least one ESD protection couple is necessaryfor functionality with many hundreds being within the scope of theinvention. Above about 20 ESD protection couples the benefit isinsufficient to justify the cost and manufacturing complexitiesassociated with the large number of layers. Below about 3 ESD protectioncouples provides insufficient redundancy. It is preferable to have about3 to about 10 ESD protection couples which is a balance betweenmanufacturing efficiencies, cost, accumulated device capacitance andeffectiveness.

The number of capacitive couples in a dual function ESD protectiondevice is not particularly limited. At least one capacitive couple isnecessary for functionality with many hundreds being within the scope ofthe invention. When the number of capacitive couples exceeds about 100the benefit is insufficient to justify the cost and manufacturingcomplexities associated with the large number of layers. Below about 3capacitive couples provides insufficient capacitance in the spaceallotted. It is preferable to have about 10 to about 20 capacitivecouples which is a balance between manufacturing efficiencies, cost andeffectiveness, although the capacitance can be tailored for specificapplication requirements.

Incorporation of an insulating material as the primary insulator layer,which is preferably an insulating dielectric material, between thedischarge electrodes and overvoltage protection element provides an ESDprotection device capable of maintaining a low effective capacitance forminimal signal distortion and low leakage to electrical ground in thepassive mode. The primary insulating material is preferably sufficientlyinsulating to minimize leakage current. It is particularly preferredthat leakage current of the ESD protection couple is no more than 5000nA, more preferably no more than 2000 nA and most preferably less than1000 nA. Furthermore, the primary insulator layer is preferably capableof withstanding the operating voltage without degradation therebyallowing the ESD protection component to return to a passive mode, afterthe transient event has subsided, without degradation. The lack ofdegradation, particularly in the primary insulator layer, improvescapacitance stability and leakage current stability even after manytransient events.

Using conventional MLCC manufacturing techniques an ESD protectiondevice can be manufactured by depositing layers of discharge electrodeand overvoltage protection element precursors directly onto a carrierfilm of ceramic dielectric precursor material. The layers can then bestacked as registered sheets followed by pressing and sintering tocreate a unified ceramic monolithic component. In this way theovervoltage protection element is separated from the dischargeelectrodes by a predetermined thickness of insulating material therebyallowing the thickness and composition to be controlled through theselection of the carrier film material. The ability to control thethickness of the primary insulator layer, coupled with the ability tocontrol the composition, allows the trigger voltage to be predictablycontrolled. Alternatively, an insulating layer may be applied as aliquid, or film, to form an insulating film layer such as a polyimide orother insulating, preferably polymeric, laminated layer.

The thickness of the primary insulator layer, or distance between thedischarge electrodes and overvoltage protection element, represented asT_(OVP) in FIGS. 3 and 4, can be much less than the typical electrodespacing in spark gap type ESD devices. Conventional spark type ESDdevices have a separation between electrodes which is typically at leastabout 6 μm and can be in excess of 50 μm. With the inventive ESDprotection device the distance between the discharge electrodes andovervoltage protection element is preferably no more than 10 μm in orderto keep the trigger voltage at which the transient is diverted to <5000Vbased on an 8 kV pulse. A thickness of the primary insulator of about 1μm is suitable for demonstration of the invention.

The trigger voltage is the voltage below which the capacitive couple, ifpresent, functions as a filter capacitor and the ESD protection coupleis passive. At, or above, the trigger voltage the ESD protection coupleshunts the excess current to electrical ground. The trigger voltage isbased on the composition and thickness of the primary insulator and thecomposition of the overvoltage protection element. As the thickness ofthe primary insulator increases the trigger voltage increases with agiven primary insulator and overvoltage protection element. It would beunderstood by one of skill in the art that the desired trigger voltagecan be obtained for a preferred primary insulator composition andovervoltage protection element by the intial preparation of multiple ESDprotection devices with a series of primary insulator thicknesses,followed by testing, to determine the optimum primary insulator layerthickness. It is desirable that the trigger voltage is at least 20%higher than the working voltage of the part wherein the working voltageis a design choice for the application as well known to those of skillin the art.

By eliminating the direct contact between the discharge electrodes andovervoltage protection element the insulation resistance is severalorders of magnitude higher than ESD devices made with an overvoltageprotection material in direct contact with the discharge electrodes. Thepresence of the discrete insulator layer, as a laminate, is instrumentalin maintaining low leakage current performance in the inactive mode overrepeated ESD pulses. The modular, multilayer nature also provides anadditional advantage in that many ESD protection couples can be stackedto form the protective element of the component thereby increasing theability of a device to withstand multiple ESD pulses before performanceis degraded.

In another embodiment of this device the discharge electrode layersshown can be alternately stacked to provide a capacitive couple withinthe component as described above. It should also be noted that it may bedesirable to use a higher permittivity (“K”) dielectric different fromthat used for the spark gap element capacitor between the terminals inorder to add capacitance to the component dependent on the speed ofsignal transmission. By controlling the capacitance of the capacitivecouple the ESD protection device can provide some noise suppression forslower transmission speeds.

To keep the capacitance low in the ESD protection couple it is desirableto reduce the overlap area between the overvoltage protection elementand the discharge electrodes. This is because the primary insulatinglayer is relatively thin. Also for the same reason it is desirable forthis insulating layer to have a low dielectric constant, preferably lessthan 100. This can be explained by applying the universal capacitanceequation to the discharge electrode coupling through the primaryinsulator wherein:C=K*K ₀ *A*n/t

-   where:-   C=Capacitance;-   K=Dielectric constant of primary insulator;-   K₀=Permittivity of free space (8.854×10⁻¹² F/m);-   A=Overlap Area of discharge electrode and overvoltage protection    element;-   n=Number of layers of discharge electrode and overvoltage protection    element; and-   t=Thickness of primary insulator.-   Therefore, at a given overlap as the primary insulator thickness    decreases the capacitance increases which can be overcome by    decreasing overlap area. The overlap protection element may itself    contain materials such as barium titanate that have relatively high    permittivity so making it desirable to minimize the overlap area.

An advantageous attribute of this inventive ESD protection device isthat the properties of the materials used in the device have anoutstanding ability to function at high operating temperatures andvoltages. An ESD protection device can be prepared which is capable ofwithstanding continuous operation at a high voltage, such as 500V and ahigh temperature such as 200° C.

The primary insulator layer, secondary insulator and gap insulator areindependently selected from those material having a low dielectricconstant and preferably each independently is, or contains, aninsulating ceramic or glass. Low permittivity dielectrics are preferredand preferably the insulating ceramic has a permittivity of no more than100, and preferably no more than 50. C0G dielectrics are particularlypreferred. Particularly preferred materials for the primary insulatinglayer, secondary insulating layer, gap insulator and capacitor ceramicinclude calcium zirconate, non-stoichiometric barium titanium oxidessuch as Ba₂Ti₉O₂₀; BaTi₄O₉; barium rare-earth oxides containingneodymium or praseodymium, titania doped with various additives, calciumtitanate, strontium titanate, zinc magnesium titanate, zirconium tintitanate and combinations thereof. As would be known to those of skillin the art the material for the primary insulating layer, secondaryinsulating layer, gap insulator and capacitor ceramic must be thermallycompatible with the discharge electrodes to avoid degradation of thedischarge electrodes during sintering of the ceramic.

The internal insulator and external insulator are not particularlylimited herein as these materials can be selected based on cost andcompatibility with the other materials. In one embodiment the internalinsulator and external insulator are the same as at least one of theprimary insulator layer, secondary insulator layer or gap insulator formanufacturing conveniences.

The overvoltage protection element comprises a conductor preferablyselected from a metal and secondary material wherein the secondarymaterial is not a conductor. The secondary material preferable comprisesat least one of a ceramic, a glass or a semi-conductor. The insulatingmaterial decreases conduction and therefore minimizes leakage current inthe ESD protection device. In the case where it is desirable to minimizecapacitance it is desirable that the overvoltage protection element doesnot appreciably overlaps the discharge electrodes. In some embodimentsthe overvoltage protection element is porous. The overvoltage protectionelement may include at least one of La, Ni, Co, Cu, Zn, Ru, Ag, Pd, Pt,W, Fe or Bi. Particularly preferred insulative ceramics include bariumtitanate or tantalum nitride. For the demonstration of this invention anovervoltage protection element consisting of 75 vol % Ni combined with25 vol % barium titanate is suitable. The metal content must be above 50vol % to no more than 90 vol %. Below 50 vol % the conductivity isinsufficient to function as an overvoltage protection element and above90 vol % the conductivity is too high to achieve adequate low leakage.It is preferred that the metal content be at least 70 vol % to no morethan 80 vol % and the secondary material represents at least 20 vol % to30 vol %.

The discharge electrodes and internal electrodes can be prepared fromany noble metal or base metal with the preference for base metals whichcan be fired in air. Preferable base metals are selected from the groupconsisting of nickel, tungsten, molybdenum, aluminum, chromium, copper,palladium, silver or an alloy thereof. Most preferable the dischargeelectrodes comprise nickel.

The ESD protection device can be manufactured in a similar manner to themanufacture of multilayered ceramic capacitors, which is well documentedand well known to those of skill in the art, wherein large discretelayers comprising printed patterns of the active layers are overlayed inregistration followed by pressing, dicing, firing and terminating toform discrete monolithic components. In the instant application theactive layers are the overvoltage protection element, dischargeelectrodes, internal electrodes and floating electrodes. It will berealized by those skilled in the art of MLCC manufacture that it isdesirable to provide cover layers consisting of blank dielectric at thetop and bottom of the device to form the external insulators describedherein thereby insulating the elements of the component from theexternal surface. The external terminals can be plated usingconventional techniques and materials and the component surface mounted.

An embodiment of the invention will be described with reference to FIG.6 wherein the process of manufacturing a ESD protection device isillustrated in flow-chart representation. In FIG. 6, a series of layersare prepared at 100. Layers 102 and 102′ comprise a precursor to theexternal insulator and are prepared in accordance with standardmanufacturing procedures for ceramic layers in MLCC capacitors as wellknown in the art. Layers 104 are those layers which, when 104A and 104Bare taken together, form the ESD protection couples after sintering. Ina preferred embodiment a layer is formed at 104A comprising anovervoltage projection element precursor and a secondary insulatorprecursor, as a coating, on a layer of primary insulator precursor. Alayer is formed at 104B comprising the precursor to the dischargeelectrodes and the gap insulator on a precursor to an internalinsulator. If capacitive couples are included a layer is formed at 105comprising alternating layers of internal electrode on ceramicdielectric precusors. A floating electrode can be formed by registrationof the same layers used to form the altering layers of internalelectrode and ceramic or it may include a different print pattern on aceramic dielectric precursor as well known in the art. The layers arestacked in registration at 106 such that discharge electrodes, insulatorlayer precursors and overvoltage protection elements are in registrationas discussed and illustrated elsewhere herein, as are the precursors tothe internal electrodes, to form a registered stack. In registration theovervoltage protection element precursor will overlay the gap insulatoras would be realized from FIGS. 1 and 2. The registered stack is pressedand heated at 108 to the extent necessary to bond the insulatorprecursors, thereby forming insulator, and to adhere adjacent layerstogether thereby forming a composite laminate sheet. The compositelaminate sheet is diced at 110 to provide discrete ESD protection deviceprecursors followed by thermal processing at 112. The ESD protectiondevice precursors are finished at 114 including the addition of externalterminations, overcoating if desired, to form the ESD protection devicewhich may be tested and packaged as part of the finishing step.

EXAMPLES

A series of 0603 EIA case size ESD protection devices were producedusing class I, C0G dielectric consisting primarily of calcium zirconatehaving a dielectric constant of about 32 as the gap insulator, primaryinsulator, secondary insulator, internal insulator and externalinsulator. The ESD protection devices were produced having varyingthicknesses of primary insulator layer. The overvoltage protectionelements comprised primarily nickel and barium titanate in a 3:1 volumeratio. The overvoltage protection elements were made with either 3 or 10coplanar pairs of discharge electrodes with each coplanar pair ofdischarge electrodes having an overvoltage protection element separatedfrom the discharge electrodes by the primary insulator layer. Theovervoltage protection elements were co-sintered within the devices. TheESD protection devices were subjected to 8 kV ESD pulses in accordancewith International Electrotechnical Commission test procedure IEC61000-4-2 and the response to the pulses were analyzed. A typical testsetup consists of a Noiseken ESS S3011/GT30R ESD simulator (150 pF 330Ωcombination) and Keysight MSOS 804A high definition oscilloscope withappropriate high bandwidth attenuation.

As discussed above, it is desirable to achieve a low trigger voltagewhilst retaining stability after exposure to multiple pulses withcontinued low leakage. To test the effectiveness of the primaryinsulating layer a multiplicity of ESD protection devices weremanufactured with 3 pairs of nickel discharge electrodes with differingthicknesses of the primary insulator layer. In Table 1 the triggervoltage after ten ESD pulses is reported as is the leakage current afterone thousand pulses.

TABLE 1 Effect of ceramic separation thickness on capacitance, triggervoltage and leakage current Ceramic Trigger Leakage separation Capaci-voltage 10 current 1000 layer/μm tance/pF pulses/V pulse/nA 0 0.44 750Short 1 0.45 750 Short 2 0.54 931 12500 3 0.52 1045 20 4 1.43 1264 7.2 50.77 1564 1

The trigger voltage of the ESD protection devices produced was evaluatedas the peak voltage recorded during repeated 8 kV point discharges. Inparts manufactured with no primary insulator layer between the dischargeelectrodes and overvoltage protection element the trigger voltage wasobserved to increase on average 25% between pulses 1-10 and 91-100. Witha primary insulator layer having a thickness of 2 μm the trigger voltageincreased an average of 13%. As the thickness of the primary insulatorlayer was increased to 5 μm the performance degradation in the triggervoltage was observed to be only 2%

ESD protection devices with 3 and 10 pairs of nickel-based dischargeelectrodes using 9 μm insulation layer thicknesses were prepared andevaluated. ESD protection devices with 10 protective layers reduced thetrigger voltage by approximately 20% whereas with 3 pairs of dischargeelectrodes leakage currents were maintained below 1 nA as shown in Table2. Furthermore, increasing the number of ESD protection couples reducedthe trigger voltage degradation over 1000 8 kV pulses from 1.7 kV to 1.3kV.

TABLE 2 Number of electrode/OVP element pairs vs Trigger voltage,leakage current and capacitance Number of V trigger after V triggerafter 1000 pulse electrode/OVP 10 8 kV 1000 8 kV leakage element pairspulses/kV pulses/kV current/nA Cap/pF 3 2.9 4.6 0.016 0.30 10 2.4 3.70.21 0.47

Voltage versus time for response is illustrated graphically in FIG. 7for an 8 kV ESD pulse. In addition to reduced peak voltage, parts withadditional ESD protection couples demonstrated improved durability withrespect to voltage response after 1000 repeated 8 kV ESD pulses, asillustrated in FIG. 8 where the average voltage versus time after 1000 8kV ESD pulsed is reported graphically.

The invention has been described with reference to the preferredembodiments without limit thereto.

The invention claimed is:
 1. A method for forming an overvoltageprotection device comprising: forming a planar first layer comprising aplanar secondary insulator precursor and a planar overvoltage protectionelement precursor on a planar primary insulator precursor; forming atleast one second layer on said planar primary insulator precursoropposite said first layer wherein said second layer comprises a pair ofdischarge electrodes and a planar gap insulator precursor between saiddischarge electrodes; forming a stack comprising said first layer inregistration with said planar overvoltage protection element precursoroverlaying said planar gap insulator precursor; and heating said stackto form a laminated stack comprising: discharge electrodes in a plane; aplanar gap insulator between said discharge electrodes within saidplane; a planar overvoltage protection element in a plane parallel tosaid discharge electrodes; and a primary insulator layer in a secondplane between said discharge electrodes and said overvoltage protectionelement wherein a pulse energy passes through said planar primaryinsulator at each transit between said overvoltage protection elementand said discharge electrodes.
 2. The method for forming an overvoltageprotection device of claim 1 wherein said planar overvoltage protectionelement comprises a conductor and a secondary material.
 3. The methodfor forming an overvoltage protection device of claim 2 wherein saidplanar overvoltage protection element has a ratio of said conductor tosaid secondary material of at least 50 vol % to no more than 90 vol %.4. The method for forming an overvoltage protection device of claim 2wherein said conductor is selected from the group consisting of La, Ni,Co, Cu, Zn, Ru, Ag, Pd, Pt, W, Fe or Bi.
 5. The method for forming anovervoltage protection device of claim 2 wherein said secondary materialis selected from the group consisting of a ceramic, a glass and asemiconductor.
 6. The method for forming an overvoltage protectiondevice of claim 5 wherein said ceramic is selected from the groupconsisting of barium titanate and tantalum nitride.
 7. The method forforming an overvoltage protection device of claim 1 wherein said planarprimary insulator layer has a permittivity of less than
 100. 8. Themethod for forming an overvoltage protection device of claim 7 whereinsaid permittivity is less than
 50. 9. The method for forming anovervoltage protection device of claim 1 wherein said planar primaryinsulator layer has a thickness of at least 1 μm to no more than 10 μm.10. The method for forming an overvoltage protection device of claim 1wherein said planar primary insulator layer comprises an insulatingceramic.
 11. The method for forming an overvoltage protection device ofclaim 10 wherein said planar primary insulator layer is selected fromthe group consisting of calcium zirconate, non-stoichiometric bariumtitanium oxide; barium rare-earth oxide; titania; calcium titanate,strontium titanate, zinc magnesium titanate, zirconium tin titanate andcombinations thereof.
 12. The method for forming an overvoltageprotection device of claim 11 wherein said non-stoichiometric bariumtitanium oxide is selected from the group consisting of Ba₂Ti₉O₂₀ orBaTi₄O₉.
 13. The method for forming an overvoltage protection device ofclaim 11 wherein said barium rare-earth oxide contains neodymium orpraseodymium.
 14. The method for forming an overvoltage protectiondevice of claim 11 wherein said titania is doped titania.
 15. The methodfor forming an overvoltage protection device of claim 1 comprisingforming a stack comprising no more than 20 first layers and secondlayers.
 16. The method for forming an overvoltage protection device ofclaim 15 comprising forming a stack comprising 3 to 10 of said firstlayers and said second layers.
 17. The method for forming an overvoltageprotection device of claim 1 wherein said discharge electrodes compriseat least one metal selected from the group consisting of nickel,tungsten, molybdenum, aluminum, chromium, copper, palladium, silver oran alloy thereof.
 18. The method for forming an overvoltage protectiondevice of claim 17 wherein said discharge electrode comprises nickel.19. The method for forming an overvoltage protection device of claim 1further comprising forming external terminations.
 20. The method forforming an overvoltage protection device of claim 1 further comprisingforming alternating layers of capacitive couple precursors prior to saidforming said stack.
 21. The method for forming an overvoltage protectiondevice of claim 20 wherein said forming said stack further comprisesoverlaying said alternating layers of capacitive couple precursors priorto said heating.
 22. The method for forming an overvoltage protectiondevice of claim 20 wherein said alternating layers of said capacitivecouple comprise at least one floating electrode precursor.